The present invention relates to solid state imagers and, more particularly, to a novel charge-injection-device (CID) imager and method of operation for realizing reduced crosstalk between adjacent rows of pixels.
A CID imager is normally operated with the magnitude of the integrated charge stored in a particular picture element (pixel) being read from that pixel before the pixel is cleared by injection of the stored charge into the underlying semiconductor substrate. While most of the pixel charge is carried into the reverse-biased P-N junction of the epitaxial layer and substrate, a portion of the injected charge nevertheless manages to move into laterally adjacent pixels. As the pixel density of a CID imager increases, the probability of charge being injected from one pixel into an adjacent pixel tends to increase. If the epitaxial layer thickness is reduced to increase the probability of capture of the injected charge by the P-N junction, the imager sensitivity begins to suffer. Thus, some form of tradeoff must be made; in a black and white vision system, the resulting crosstalk leads to a reduction of the apparent resolution, thus defeating the purpose for attempting higher pixel density. In a color imager system, the effect is even more problematic as the photons passing through one color filter may end up in another channel and cause color crosstalk which distorts the color rendition of the frame. While several techniques for reducing the actual crosstalk, or the crosstalk effects, have been proposed, such techniques not only require additional imager processing or additional circuitry, but also provide only a partial crosstalk reduction. Those techniques which reduce crosstalk effect by mathematically performing crosstalk inversion suffer from the drawback that the exact amount of crosstalk of each location must be known, even if the additional circuitry required to perform the calculation is not an undue addition to the imager.
Prior to this invention, a known CID imager was read-out by having a first row of pixels read into a single line store, or row delay, means at a first time. Then all pixels along that row are cleared in parallel by injection, with some portion of the injected charge ending up in the pixels of an adjacent immediately-previous and immediately-subsequent imager pixel row. In the next time interval, the now-empty pixels along the first row and the pixels along the immediately-subsequent row are simultaneously read out, with a differential amplifier taking the difference between the delayed first row signal (from the single line store) and the now-empty first row signal; the signal subtraction removes fixed pattern noise. The second full row signal is read into the delay means; the delayed signal now consists of the actual second row signal plus a crosstalk portion attributable to injection of the first row signal. In a subsequent third time interval, the second row is cleared by injection, with the second row charge crosstalking into pixels of the first row and an adjacent third row of imaging cells. The first row may be held in the injected state during this time interval to eliminate crosstalk from the second row back to the first row. The third row and the now-empty second row are then read out, with the now-empty second row signal being subtracted from the delayed second row signal, to remove fixed pattern noise from the output signal attributable to the second pixel row. The process cyclically continues across all rows of the image. However, significant crosstalk contribution may occur.
It is therefore highly desirable to reduce injection crosstalk in a CID imager, while still maintaining high sensitivity and relative simplicity of readout circuitry.